mkdir -p files/{css,js/{custom,external},fonts}

The above command will create a file structure as shown below. Cool if you need to create the same folder structure over and over - or dynamically. The -p stands for parent and the flag is defined as no error if existing, make parent directories as needed in the man pages.

└── files
    ├── css
    ├── fonts
    └── js
        ├── custom
        └── external